Various methods have been proposed to form droplets of material having a known size. The present invention is an improvement over the method described in the Chun et al. U.S. Pat. No. 5,266,098 which is expressly incorporated herein by reference. The Chun et al. '098 describes placing a charge on a droplet in an attempt to produce uniformly sized droplets. The Chun et al. method produced a high percent of unacceptable and unusable pieces or bits of material having flat, oblong or irregular shapes. The Chun et al. method is limited to producing irregularly shaped droplets having undesirably rough and bumpy surfaces.
Other previously considered methods for forming droplets by imparting a charge on the droplets include: Smith U.S. Pat. No. 5,560,543 describes a method for forming droplets and passing droplets through charged and grounded plates to selectively deflect droplets. Soviet Patent No. 541682039-A1 describes forming droplets which acquire a charge by passing through an electric field. Orme et al. U.S. Pat. Nos. 5,171,360; 5,226,948; 5,259,593 and 5,340,090 describe methods and apparatuses for forming a net product by directing a stream of uniformly sized droplets onto a collector of the shape of the desired product. C. H. Passow thesis, The Massachusetts Institute of Technology (MIT) May 1992, describes forming uniform droplets by using parallel plates positioned below a charging plate to selectively deflect some droplets off to one side where they are collected.
Various other methods proposed to form droplets include: Hayes U.S. Pat. No. 5,411,602 describes ejecting solder drops from an ejection device into a flow of inert gas and catching the solidified solder balls; Hommel et al. U.S. Pat. No. 4,956,128 describes an aqueous calcium chloride hardening solution through which droplets are passed; Yabuki et al. U.S. Pat. No. 4,744,821 describes forming drops and passing the drops through layers of oil and water; Fulwyler et al. U.S. Pat. No. 4,302,166 describes a droplet forming apparatus where the droplets fall into an aqueous solution of a nonionic surfactant; Green et al. U.S. Pat. No. 4,628,040 describes forming droplets using a venturi process where the droplets pass through an oil to harden the droplets; Eylon et al. U.S. Pat. No. 4,787,935 describes a method for making powders using swirling, cooling fluids to harden the droplets; Anderson U.S. Pat. Nos. 4,216,718 and 4,419,303 describe forming sodium amalgam particles for high pressure discharge lamps where a vibrating discharge nozzle forms droplets which fall into a fluid; Rhim et al. U.S. Pat. No. 4,981,625 describes forming polymeric microspheres by ejecting droplets of monomers, charging the droplets, freezing the droplets in a cryogenic liquid, and thawing the droplets by irradiation to activate free radicals which polymerize the monomer.
The prior art droplet formation methods have not been entirely satisfactory for a number of reasons. A major concern is the wide distribution of the particle size of the droplets. Inconsistent sizes in the droplets makes the use of the droplets more difficult in soldering applications. Another problem is that after the formation of the droplets, the droplets have to be cleaned to remove contaminants or oxides on the surface or to remove oils and solutions through which the droplets have been passed. The cleaning of the droplets adds to the manufacturing time and costs.
Another major concern is that the droplets have irregular shapes and/or bumpy or rough surfaces. This lack of sphericity makes handling and use of the individual droplets more difficult.
Therefore, it is an object of the present invention to develop an apparatus and method for manufacturing high quality uniformly sized and shaped droplets. The present invention further provides a process which does not involve the use of multiple formation steps and/or cleaning steps.